1. Field of the Invention
The present invention relates to devices for controlling the flow of a fluid through a conduit, and particularly to a system and method for controlling flow characteristics of a fluid through a valve.
2. Description of the Related Art
In many types of fluid systems, it is necessary to modulate or otherwise control a flow of fluid through a fluid circuit. For example, heating, ventilation and air conditioning (HVAC) systems for buildings typically include multiple fluid circuits providing heated or chilled refrigerant to heat exchangers for controlling the temperature within the building. The flow of fluid in the circuits is generally modulated by one or more valves in each fluid circuit, which are operatively connected to and controlled by a control device, such as a thermostat, to provide a flow of fluid to the heat exchangers, which will result in a desired temperature being maintained in inside the building.
As part of the process of selecting a control valve for use in a particular fluid system, the relationship between the effective flow area of the control valve relative to the degree of opening is known as the “valve characteristic.” For valves used in HVAC systems, it is typically desirable to have a so-called “equal percentage” valve characteristic over a desired range of fluid flows. In a valve exhibiting equal percentage characteristics, a change in the degree of opening of the valve (as a percentage) with respect to a previous given degree of opening will result in an equal percentage change in fluid flow over the fluid flow at the previous given degree of opening. For example, if opening the valve by an additional 10% causes a corresponding 10% increase in fluid flow, the valve exhibits equal percentage characteristics. A valve with equal percentage flow characteristics increases fluid flow at a very low rate when the valve first begins to open. As the degree of opening becomes larger, the rate of increase in fluid flow for an incremental increase in opening becomes larger.
For more generalized systems, when a valve position is determined and controlled by the controller, which directly regulates the valve based upon a particular quantity, such as temperature, pressure, fluid level, etc., the valve flow characteristics appear in cascade with the process transfer function characteristics. Proper selection of the valve flow characteristic becomes necessary in order to cancel out effects of process nonlinearities for smooth regulation and control of the desired process variable. The primary function of the control valve is regulating the process variable, rather than regulation of the flow at a desired flow rate. Typical valve controllers are mechanical devices, either relying on manual resetting for particular characteristics, or with only a limited number of mechanical positions available. Such valve control devices do not allow for variation in valve characteristics, particularly for fluid flow systems in which the user needs to carefully and precisely control fluid flow.
None of the above inventions, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, a system and method for controlling flow characteristics solving the aforementioned problems is desired.